The development of a high-output and high-energy density battery for each power source of an electric vehicle and a hybrid powered vehicle as an automobile in which care is given to environment is demanded. As the voltage is high and the energy density is high as to a lithium secondary battery using non-aqueous electrolytic solution as a battery used for these, the development is energetically advanced in each field. Besides, as for a battery for an automobile, a long life characteristic, stable voltage controllability, resistance to environment, large-sizing and the reduction of the cost are demanded in addition to characteristics of a conventional type private battery.
As for a battery for an automobile, the secure operation in a wide temperature range from low temperature to high temperature is demanded. Particularly, as the electric conductivity of a lithium ion in electrolytic solution is deteriorated at low temperature and the characteristics of the battery are greatly deteriorated, it is proposed that the discharge rate of the battery is improved.
The form of a particle of lithium transition metal composite oxide which is the material of a positive electrode is closely related to the discharge rate. Particularly, as the specific surface of positive electrode material is related to the area of a solid-liquid interface on which the positive electrode material and electrolytic solution act, it has an effect upon the discharge rate. Therefore, in JP-A 245106/1995, a lithium secondary battery made excellent in a high discharge rate characteristic by using positive electrode material the specific surface of which is high is proposed.
In the meantime, in JP-A 37576/1995, positive electrode material provided with large capacity and made excellent in charging/discharging efficiency by a secondary particle having particle structure that the layer-structure surface of a monocrystalline particle of positive electrode material having a crystal of layer structure is exposed outside is proposed.
Besides, in JP-A 85006/2001, the lithium secondary battery capacity of which is large and which is made satisfactory in a cycle characteristic by defining the voidage in a secondary particle of positive electrode material so as to optimize particle structure in lithium-nickel composite oxide is proposed.
Secure operation in a wide temperature range from low temperature to high temperature is demanded for a battery for an automobile. When a lithium secondary battery is operated at low temperature, the ionic conductivity of electrolytic solution is extremely deteriorated at low temperature. For example, in a conventional type battery, the ionic conductivity at −30° C. of electrolytic solution including LiPF6 which is electrolytic solution in a mixed solvent of ethyl carbonate (EC) and dimethyl carbonate (DMC) is approximately ¼ of that at room temperature. Therefore, even a lithium secondary battery that acquires a sufficient characteristic at room temperature can acquire only a low battery characteristic at low temperature. Besides, as long life is demanded for a battery for an automobile, the maintenance of a cycle characteristic is also essential at the same time as a low-temperature characteristic.
In JP-A 245106/1995, the effect to some extent is recognized at room temperature, however, in the environment of low temperature, a discharge rate characteristic demanded for an electric vehicle or a hybrid electric vehicle is insufficient. Besides, in JP-A 37576/1995, a discharge rate characteristic at low temperature is also insufficient in the above-mentioned particle structure. In the meantime, as for the positive electrode material disclosed in JP-A 85006/2001, as the quantity of Ni included in transition metal is 50% or more, the expansion/the contraction caused by charge/discharge of a crystal lattice is large. The cycle characteristic is enhanced by increasing the voidage in the secondary particle up to 10% or more from a value below 10% so as to reduce the effect of the change in volume of the lattice in the composition. Besides, the density of positive electrode material is enhanced by setting the voidage to 30% or less and service capacity is increased. However, the discharge rate at low temperature is insufficient.